When utilizing directed infrared countermeasure (DIRCM) systems, the systems employ a gimbal pointing system which is used in the tracking and lazing of a target once it has been detected. The gimbal tracking is provided by a DIRCM gimbal head that has pointing mirrors and an embedded IR camera. The camera has a frame rate and integrates the collected data during which the camera is temporarily shut-off. During this period the gimbal pointing system aim-point is prone to any direct or induced gimbal movement. One of the largest angular movements occurs when the aircraft rolls, pitches or yaws, oftentimes up to 400 degrees per second. Because the DIRCM head is mounted external to the aircraft, the aim-point can be severely impacted during camera integration time. The other major contributor to angular movement is induced aircraft vibration. Thus during integration time the aim-point is susceptible to movement if no compensation system is employed. The problem therefore becomes how to counteract the motion of the aircraft when utilizing a directed countermeasure system to illuminate a target.
In a “strapped down” approach to provide stability in a gimbal pointing system, the gimbal motion is sensed at its base by a gyroscope, and drives are provided to cancel out the sensed angular motion. However, these systems utilize complicated algorithms to sense gimbal platform motion.
In particular, if one were to mount a gyro stabilization unit at the base of a DIRCM gimbal head, the distance between the sense mechanism and the optical elements in the gimbal head present problems resulting in aim-point errors. To eliminate the interpolation and complicated algorithms used in gimbal stabilization, designers moved the gyro closer to the input mirror carried by the DIRCM head. However the sheer weight of a gyro when strapped-on to the optical members of the gimbal generates noise due to the flywheels themselves, thus limiting the success of the stabilization process.
Note that there are stringent pointing accuracy requirements for the gimbal systems used for IR countermeasures which necessitates the use of a high gain tracking loop. The requisite aiming accuracy is on the order of 200 microradians which in itself is difficult to achieve. Add to this aircraft motion and the task of providing 200 microradian accuracies is challenging. Attempts have been made to provide stabilization using two axis gyroscopes. However, it was discovered that two axis gyroscopes could not support the high gain track loops. Simply put, the cross coupling inherent in the gyroscope nutation dynamics does not allow the nutation mode to be satisfactorily stabilized.
To realize the required performance, tracking loops require gains on the order of 8,000 sec−2. Laboratory experiments have indicated the onset of nutation mode instability at gains exceeding 400 sec−2; and with control loop redesign one can only achieve stable operations with gains below 1,000 sec−2. Since the aim-point error accuracies of 200 microradians require track loops gains on the order of 8,000 sec−2, something other than gyro stabilized platforms is required.
It is therefore necessary to provide better stability, particularly for two axis gimbal pointing systems, so that infrared laser radiation from an infrared laser mounted to the gimbal actually intercepts and impinges on the target.
More particularly, the gimbaling platform for a DIRCM system is subjected to vibration and accelerations due to aircraft movement, and importantly during the course of a camera frame. In target acquisition involving the detection of the target on a focal plane array carried by the camera, the camera is turned on and off at a predetermined frame rate. During the off time a considerable amount of aircraft movement can occur which moves the gimbal aim-point.
In the past, stabilization systems have locked down the tracking system when the camera is off, and this is accomplished by inhibiting stabilization signals. During lockdown the gimbal pointing system continues to point the laser beam at the last computed point. If the aircraft pitches, rolls or yaws when the camera is on, pointing stabilization corrections are applied. However, when the camera is off the system tracks normally, but without stabilization. There is therefore a need to provide stabilization when the camera is off.
As mentioned above, some systems put the stabilization gyro at the base of the gimbal to monitor the movement at the gimbal pointing system base. This however is ineffectual because the translation or interpolation problem in referencing the movement at the gimbal base to where the gimbal pointing mirrors are located. Thus, the gyro-stabilized gimbaling systems monitor motion at some distance from the pointing mirrors. Due to the distance between the sensing platform and the mirrors, the error could be a substantial half laser beam width, or as much as 500 microradians. Given the 200 microradian requirement, gyro stabilization translation errors preclude the use of gyros. Moreover, if it were possible to place the gyro at the mirror location, thus eliminating the problem of interpolation, noise caused by the nutating masses of the gyroscope is nonetheless injected into the system.